This invention relates to a process and apparatus for treating whole blood either to separate out plasma or to oxygenate the blood in a manner to permit continuous recirculation of the treated whole blood to the donor.
There are available, at the present time, a wide variety of blood oxygenation apparatus and plasmapheresis apparatus.
Present plasmapheresis devices separate the whole blood of a blood donor in order to recover the plasma which then can be given to a patient requiring a transfusion. Plasmapheresis devices also are utilized to remove the plasma from the whole blood of a patient suffering from diseases associated with an excessively active immunological system that produce excess antibodies such as systemic lupus erythematosus, myasthenia gravis, or Goodpasture's syndrome. In either instance, it is desirable to return the concentrated whole blood to the patient or the donor since, in the former instance, such practice permits the donor to give blood again within a period time about one quarter of that required when the whole blood is donated. In the latter instance, such a practice permits treating the patient's entire blood volume in a single treatment. In both instances, the advantages of such a practice are obvious since it permits either an increased quantity of donated blood per donor or increased effective treatment of a dangerous disease.
It has been proposed to provide a method and apparatus for extracting blood to form a plasma fraction and an enriched blood fraction in U.S. Pat. No. 3,705,100. The apparatus utilizes a reservoir for whole blood and a filtration membrane as well as a flow directing means and a pressure generating means for passing the blood adjacent the membrane, thereby effecting the desired filtration. The primary disadvantage of this device is that it is a batch device and it is incapable of continuously treating blood from a patient and for continuously recirculating enriched blood to the patient. This, of course, is undesirable since it greatly increases the time required for treating a given volume of blood and subjects the patient to multiple punctures in order to obtain samples to be treated.
It also has been disclosed in the final report of NHLBI Contract No. 1-HB-6-2928, June, 1976-April, 1979, to the American Red Cross to utilize a blood filtration system that permits continuous withdrawal of blood and continuous reintroduction of enriched blood back to the patient. However, this device requires a recirculating system for passing blood through a channel and adjacent filter a multiplicity of times. The recirculating system is required to achieve proper balance between shear forces on the blood and pressure drop for the channel height utilized, thereby to minimize blood damage. While the desired balance is achieved, the increased surface exposure for the recirculating blood necessitated thereby also increases the risk of blood damage thereby rendering the apparatus undesirable. In addition, U.S. Pat. No. 4,191,182 to Popovich discloses a plasmapheresis apparatus that requires an undesirable recirculating system for blood in order to maintain the perceived required shear stresses during filtration.
Membrane oxygenators are available wherein blood and oxygen-containing gas are passed into contact on opposing surface of a membrane and wherein oxygen is transferred through the membrane into the blood while carbon dioxide is transferred from the blood through the membrane into the oxygen-containing stream. Presently available membrane oxygenators provide a relatively thick blood film of generally greater than 0.2 mm. The factor limiting efficiency in such devices is resistance to oxygen diffusion in the blood film.
One approach to reducing this resistance is typified by U.S. Pat. No. 4,168,293 in which a woven screen is introduced into the blood channel to induce mixing of the blood. This approach, however, leads to blood damage and potential thrombus formation, especially if used for long-term respiratory support.
Another approach is typified by Bellhouse et al, Transactions of American Society of Artificial Internal Organs, Volume XIX, 1973, page 72, which describes a furrowed membrane and a pulsatile pumping system also intended to create mixing within the blood channels. While potentially less damaging to the blood than a mixing screen, the device and its associated hardware are complex and costly to manufacture. The 0.4 mm blood film thickness of this device is what leads to the necessity for such measures.
It would be highly desirable to provide a membrane device for processing blood whose blood channels approached the dimensions of the microcirculation of the human body (less than 0.1 mm diameter or height). The high surface to volume ratio of such a device would allow the separation of blood into a plasma stream and an enriched blood stream to be reintroduced into the patient in a single pass through the device. Such a device would minimize both blood damage and blood priming volume.
Utilized as a blood oxygenator, the high surface to volume ratio would give inherently high efficiency. Further increase in efficiency results from shear augmentation of oxygen diffusion without the need for blood damaging mixing screens or elaborate externally driven systems.